Scrapy provides a convenient facility for collecting stats in the form of
key/values, where values are often counters. The facility is called the Stats
Collector, and can be accessed through the stats
attribute of the Crawler API, as illustrated by the examples in
the Common Stats Collector uses section below.

However, the Stats Collector is always available, so you can always import it
in your module and use its API (to increment or set new stat keys), regardless
of whether the stats collection is enabled or not. If it’s disabled, the API
will still work but it won’t collect anything. This is aimed at simplifying the
stats collector usage: you should spend no more than one line of code for
collecting stats in your spider, Scrapy extension, or whatever code you’re
using the Stats Collector from.

Another feature of the Stats Collector is that it’s very efficient (when
enabled) and extremely efficient (almost unnoticeable) when disabled.

The Stats Collector keeps a stats table per open spider which is automatically
opened when the spider is opened, and closed when the spider is closed.

Besides the basic StatsCollector there are other Stats Collectors
available in Scrapy which extend the basic Stats Collector. You can select
which Stats Collector to use through the STATS_CLASS setting. The
default Stats Collector used is the MemoryStatsCollector.

A simple stats collector that keeps the stats of the last scraping run (for
each spider) in memory, after they’re closed. The stats can be accessed
through the spider_stats attribute, which is a dict keyed by spider
domain name.

A Stats collector which does nothing but is very efficient (because it does
nothing). This stats collector can be set via the STATS_CLASS
setting, to disable stats collect in order to improve performance. However,
the performance penalty of stats collection is usually marginal compared to
other Scrapy workload like parsing pages.